1. Field of the Invention
Broadly, the invention relates to an improved apparatus and method for mixing dry particles with a liquid. Specifically, the invention is directed to apparatus and method which is particularly suitable for both practice simulation and actual use in mixing and recirculating dry cement with water to obtain cement slurries of desired density for use in a particular oil well cementing operation.
2. Background
Utilization of cement within oil wells, particularly, in the cementing of casing therein has been under development since the early 1900's. Two of the purposes of placing cement into the annular space between the casing and the formation are: 1) to support the casing within the well, and 2) to seal off undesirable formation fluids.
Casing is typically secured in the well bore by the cement being mixed at the surface by being pumped down the open center of the casing string and thence back up the annular space which exists between the outer diameter of the casing and the inner diameter of the oil well bore. A displacement fluid, such as drilling mud, is pumped behind the cement to push the cement to the desired location. In many oil and gas well applications it is often necessary to provide cement mixers which will rapidly prepare large quantities of material to be pumped into the well by a batch or continuous process until a sufficient predetermined quantity has been applied. In either case, the process usually begins with the material being pre-prepared by dry blending and water being added at the well site. Batch mixing is one form of system to obtain a satisfactory slurry, but batch mixing requires an initial outlay of a large amount of equipment, people and space. In offshore operations, space and weight capacity are expensive. Batch mixers use valuable space and add to rig weight. Typically, large tanks with rotary paddle type mixers, although being able to adequately perform the mixing operations, have not been efficient in terms of space, numbers of people required or equipment costs where large volumes of mixing must be done at the well site.
For the continuous process, there must be continuous monitoring of and adjustments to the mixed slurry in order to insure that it will have the proper qualities and characteristics once it has been placed into the well and into the annular space between the casing and the well bore.
Probably one of the most critical elements of oil well cementing is the maintenance of the required density and the capability of changing that density during the cementing operation as needed. One quality measurement of a cement slurry is its conformance to the desired density. Thus, the density must be controlled especially where the cement will be positioned opposite producible geologic formations which will need to be perforated so that the oil or gas from the zone or zones will flow into the casing for production. Density of the cement mixture may have differing characteristics at different well sites of geological zones, i.e., it must be suitable for the downhole environment where it is to be used. For example, varying depths, downhole pressures, temperatures and geological formations may call for cement slurries of different densities. In other instances, it may be necessary to utilize cement of a particular density to seal off a water table encountered in the well bore, or there may be porous formations or cavities encountered which may need to be filled and plugged requiring the use of other additives or fillers during the cementing process. As a result, these factors require the density and makeup of the cement to be constantly monitored and controlled. All of these characteristics must be designed and accounted for, typically at the well site during the makeup of the cement slurry.
Slurry density is controlled by adjusting the ratio of cement dry blends and mix water. If the bulk blend is constant, a less than required amount of water can result in too high density and result in an insufficient volume of slurry being placed into the well. Also, viscosity of the slurry will be high and, therefore, pumping pressures may be excessive and could cause a loss of circulation in certain formations. The quality of the cement slurry placement process involves the completeness of the mixing process and the pumping rate which can affect the bond between the casing and the well bore. In addition, cement and additives such as loss circulation materials and weighting materials need to be thoroughly mixed to prevent separation or premature setting.
Many types of cement mixers have been known in the prior art. For example, jet-type mixers and vortex mixers such as those disclosed in U.S. Pat. Nos. 3,201,093 and 3,741,533 have been used with considerable success but have not necessarily been successful in continuously mixing cement slurries while maintaining substantially constant density, or quickly changeable density for different application during the cementing of the oil well casing. Such jet or eductor type mixers worked reasonably well when slurry designs were simple. With the more enhanced slurry designs of today, the jet mixer cannot adequately mix these slurries and does not allow adequate density control for today's specified tolerances.
Continuous recirculating mixers were developed to overcome some of the deficiencies of the jet type and batch mixers. These systems mix dry cement and water in an inlet mixer, the output going to a tank for agitation with excess slurry flowing over a weir to an adjustment tank, which may be agitated, thence pumped into the well. Typically, a portion of the mixed slurry was recirculated from the mixing tank and directed back into a modified jet mixer. Thus, newly delivered dry bulk cement was wetted both by water and recirculated cement. This provided additional mixing energy that enabled the satisfactory mixing of higher slurry densities. These type mixers were first introduced during the early 1970's. Since that time, cement slurry design has evolved into the use of more complex slurries that continuous mixing systems are unable to achieve. Thixotropic slurries with very low "free water" requirements have evolved for the deep, high temperature, high pressure gas wells. It seems as though the industry is constantly testing the ability of mixers by developing even more difficult to mix slurries. Furthermore, tighter tolerances on slurry density control are being developed. Density, however, cannot be controlled if the mixing process is not adequate. Hence, a satisfactory mixing means is the key to successful control over slurry density in a continuous process.